Apparatus and method for bonding tubular elements



Aug. 21, 1962 l. SHEINHARTZ ETAL 3,050,613

APPARATUS AND METHOD FOR BONDING TUBULAR ELEMENTS Filed Sept. 23, 1959 INVENTORS A Sl/fil/WM T y 4/. L. Ema/ ow ,4 TTOPNEYS United States Patent fitice 3,050,613 Patented Aug. 21, 1962 3,050,613 APPARATUS AND METHOD FOR BONDING TUBULAR ELEMENTS Irving Sheinhartz, Whitestone, and John L. Zambrow, Westbury, N.Y., assignors, by mesne assignments, to Sylvania Electric Products Inc., a corporation of Delaware Filed Sept. 23, 1959, Ser. No. 841,819 Claims. (Cl. 21950) This invention relates to tubular nuclear fuel assemblies and more particularly to the apparatus and methods for the bonding of the core and cladding elements of such assemblies.

The fuel elements in nuclear power devices comprise plates, tubes or other shapes fabricated of uranium alloys or other radioactive materials capable of emitting large quantities of neutrons. The fuel elements are commonly immersed in a circulating heat transfer medium, and in order to protect said elements from erosion and yet provide good thermal conductivity to the said circulating medium, it has been found desirable in the art to clad said fuel elements with metal protective armor.

Aluminum and stainless steels have been found to be excellent armor-cladding materials because of their erosion resistance, as well as their good thermal conductivity and low atomic cross-section.

However, the cladding of uranium alloys with aluminum or stainless steel has presented several practical difficulties. These difficulties have been best avoided in the manufacture of cladded fuel element sheets or plates by the use of the so-called picture frame method of fabrication.

The picture frame method of fabrication comprises the production of a uranium alloy core sheet, and a larger aluminum or stainless steel sheet of the same thickness. The larger sheet has a centrally located opening, the size and shape of the uranium core sheet. The uranium core sheet is fitted into the larger sheet which forms the socalled picture frame. Two other sheets of the same material and of the same size as the first larger sheet, but Without centrally located openings, are then placed sandwich-like about the above picture frame composite sheet. The total composite is then hot rolled to assure a good metallurgical bond. Flux annealing must be employed at this stage to avoid blisters in the sandwich. Finally, clad rolling reduces the sandwich to its final dimensions.

While the above method has proven satisfactory, though tedious, for the fabrication of cladded uranium sheets and plates, no equally satisfactory direct method has been evolved in the art to directly fabricate cladded uranium tubular fuel elements.

Consequently, the best method for producing such tubular fuel elements has been to produce a cladded plate by the picture frame method just as described above, and then to roll said sheet into a tube and seal the butt seam created at the opposed edges meeting line. This method of fabricating tubular fuel element tubes is complex and expensive. Its avoidance as a first step in the fabrication of such elements would save considerable time and expense.

Other disadvantages are more important. The uranium core does not extend completely from edge to edge in the rolled tube, particularly at the butt point and consequently a considerable portion of the circumferential area of the completed tube contains no uranium layer. This creates a blind spot of neutron emission, disturbing uniformity and is also a needless waste of productive reactor space, diminishing compactness accordingly. Also, the rolled tube must be sealed along the butt seam resulting in an additional fabrication operation, and an inspection operation to guarantee that it is leakproof.

The seal may warp the assembly, as where a brazed seal is used and moderators such as lithium contained in the flux may remain in voids in the seal.

Another disadvantage resides in the fact that it is difficult to control the thickness or the width of the rolled plates closely. This makes it difficult to maintain tolerances on the diameter of the resulting tube and the thickness of its wall. The circularity produced by the rolling operation is also difiicult to control.

Thus, there has long been a need in the art for a method of fabrication that would avoid the above difiiculties in tubular clad core elements. The instant invention has answered this need in the art.

Briefly stated, the invention contemplates the manufacture of tubular elements by the use of a cylindrical mandrel and a tubular die, one or both of which is subjected to heat to produce a temperature differential resulting in a differential expansion.

A raw fuel element is readied by slipping a uranium tube over an aluminum or stainless steel first cladding tube whose outside diameter is the same as the inside diameter of said uranium tube. A second cladding tube of the same material as the first, but whose inside diameter is the same as the outside diameter of said uranium tube is then slipped over the assembly of the uranium and the first tubes. A tight fitting three-layer tube is thus presented, comprising a uranium layer between two aluminum layers or two stainless steel layers.

The outer surface of said three-layer tube is then tightly received by a tubular die, and the inner surface receives a close-fitting cylindrical mandrel. By means of heat, the mandrel and die are caused to differentially expand, that is, the mandrel is thermally expanded with regard to the die. The three layers of the tube are thereupon mechanically and metallurgically locked together. Differential expansion can result from cooling the die and heating the mandrel, or from heating both while employing a mandrel material of an appreciably higher coefiicient of expansion than the material of the die.

The product of this process is a highly circular, seamless, continuous core tube, whose outside and inside dimensions can be closely controlled. The tubes can be made with speed, economy and simplicity.

It is an object of this invention to provide a simple and inexpensive method for fabricating clad uranium core tubular fuel elements.

Another object is to provide a method utilizing a simple, inexpensive mandrel-die apparatus for such fabrication.

Another object is to provide a method for fabricating tubular clad fuel elements of seamless construction.

A further object is to provide a method for fabricating tubular clad fuel elements wherein thefuel layer is continuous around the circumference of said tube.

A still further object is to provide a method for fabricating tubular clad fuel elements adapted to permit a high degree of control of dimensional tolerances.

Further objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of one embodiment of the die assembly including a ceramic or other liner and cooling means;

FIGURE 2 is a perspective view of a clad uranium fuel element tube before processing, and alternatively of a completed tube assembly;

FIGURE 3 is a perspective view of one embodiment of a mandrel with internal heating means;

FIGURE 4 is a perspective view of a second embodiment of the die assembly including a ceramic or other liner; and

FIGURE 5 is a perspective view of a second embodiment of a mandrel.

Referring to the drawings, the invention comprises a die assembly 10, a raw fuel element assembly 20 and a mandrel 30, the fuel element assembly 20 being disposed to fit tightly into the die assembly 10, and the mandrel 30 disposed to fit tightly into the fuel element assembly, the mandrel being disposed to expand relative to the die upon heating so as to upset and bond each of the layers of said fuel assembly by mechanical and thermal action.

In FlGURE l, the die assembly comprises a body portion 11, a liner portion 12 and cooling means 13.

The die body portion 11 is fabricated of steel preferably, but any durable metal capable of sustaining high hoop stresses may be substituted. For the centrally located opening in body portion 11, We prefer a cylindrical configuration of circular cross-section, though other configurations may be employed so long as they cooperate in a tight fit with a similar outside surface configuration of line portion 12.

Liner portion 12 may have any outer surface configuration that tightly matches the configuration of said centrally located opening in body portion 11, though we prefer a matching circular cross-section cylindrical configuration for both. The inner surface of liner portion 12 must describe a cylinder of circular cross-section and must afford a close fit to raw fuel element assembly as hereinafter described. Liner portion 12 completely lines said opening in body member 11 and is fabricated from refractory material of high hardness and compressive strength, preferably a ceramic.

Cooling means 13 is utilized in die 10 and may comprise a continuously circulating cooling coil, as shown in FIGURE 1, or a constantly replenished water jacket, or any other suitable means disposed to cool body portion 11.

As shown in FIGURE 2, raw fuel element assembly 20 comprises a tubular uranium fuel core 21, a tubular inner aluminum or stainless steel liner 2?, and a tubular outer liner 23 of the same material as the inner liner. Said inner liner 22 is disposed to fit tightly into said fuel core 21, and said outer liner 23 is disposed to fit tightly around said fuel core 21.

As shown in FIGURE 3, mandrel comprises a cylindrical working portion 31, an insertion and extraction portion 32, and a centrally located opening extending nearly to the end of said working portion 31 opposite said insertion and extraction portion 32 and containing heating means 33. We prefer to fabricate said mandrel in steel, and prefer electrical resistance coil heating means, though other modes may be used to heat working member 31, as by conduction from member 32. Electrical means are preferred because they afford even heating and, therefore, even diametrical expansion of the mandrel.

In a second die and mandrel embodiment as shown in FIGURES 4 and 5, die cooling means 13 and mandrel heating means 33 are both omitted. The differential expansion of the mandrel and die in the second embodiment results from their different thermal coefficients of expansion, rather than from being subjected to differential heating as in the first embodiment.

As shown in FIGURE 4, die comprises a body portion 41 fabricated of a material having a very low thermal coefficient of expansion such as a reinforced ceramic or graphite, or a metal such as die steel. A liner 4-2 is disposed within body portion 41, and is a duplicate of liner 12 of the first embodiment.

As shown in FIGURE 5, mandrel comprises an insertion and extraction portion 52, and a solid cylindrical Working portion 51 fabricated of a material having a very high thermal coefficient of expansion such as an austenitic stainless steel containing over 7% nickel and 17-18% chromium.

In operation, in both embodiments, liner 22. is forced into fuel core 21 and outer liner 23 is forced over fuel core 21, The three parts fit closely together, but no more 4 force is required to mate them than can be exerted by hand.

The resulting assembly 20 is then hand forced into die assembly 10 or 4-0 until it is completely encased therein. A suitable lubricant, as, for example, graphite, may be used if required; such lubricant may facilitate removal of the assembly 20 after the bonding operation is completed.

Mandrel 30 or 50 is then inserted in fuel element assembly 20 within the die.

In the first embodiment, heating means 33 within the mandrel and cooling means 12 associated with die assembly 10 are then activated. The mandrel is heated to approximately 600 C. if aluminum is the cladding element, and to approximately 1200 C. if stainless steel is the cladding element, and said die assembly is kept approximately at room temperature by cooling means. The aforesaid temperatures are chosen because they are just below the approximate melting point of the cladding material or elements. The differential expansion of the mandrel and die assembly, together with the temperature thermal effect upsets and bonds together the three layers of the fuel element assembly mechanically and metallurgically. After a suitable time, for example, after one hour at temperature, the mandrel is cooled, and removed and the finished fuel element assembly 20 is removed from the die.

In the second embodiment, the entire assembly of mandrel 50, fuel element 20 and die member 40 is placed in an oven at approximately 600 C., if the cladding element is aluminum, or at approximately 1200" C. if the cladding element is stainless steel, and the differential ex pansion of the mandrel and die member, caused by the different rates of expansion of their constituted materials, duplicates the effect above described in said first embodiment. Freeing of said finished and bonded fuel assembly is the same as above.

Either method and embodiment is described above achieves the desired objects, that is, a seamless tube with a continuous uranium core layer, produced economically and with high tolerances.

Although the invention has been described with a certain degree of particularity, it is to be understood that the present disclosure is by way of example and that changes in the details of construction and operation can be made without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A device for mutually bonding a plurality of coaxially fitted metal tubes comprising outer means closely surrounding the outermost of said tubes, inner means in close registration within the innermost of said tubes, and means adapted to thermally expand said inner means relative to said outer means.

2. A device for mutually bonding a plurality of coaxially fitted metal tubes comprising outer means closely surrounding the outermost of said tubes, inner means in close registration within the innermost of said tubes, and means adapted to thermally expand said inner means more than said outer means.

3. A device for mutually bonding a plurality of co axially fitted metal tubes comprising a die having a hollow body portion closely fitting the outermost of said tubes, means adapted to maintain the dimensions of said die body portion under thermal stress, a mandrel closely fitting the innermost of said tubes, and means adapted to thermally expand said mandrel.

4. A device for mutually bonding a plurality of coaxially fitted metal tubes comprising a die having a hollow body portion closely fitting the outermost of said tubes, fluid cooling means adapted to cool said die body portion, a mandrel closely fitting the innermost of said tubes, and electrical resistance coils adapted to heat said mandrel.

5. A device for mutually bonding a plurality of coaxially fitted metal tubes comprising an insulating tubular sleeve of relatively hard material closely fitting the outermost of said tubes, a die having a hollow body portion closely fitting the outer surface of said insulating sleeve, cooling means adapted to cool said die body portion, a mandrel closely fit-ting the innermost of said tubes, and heating means adapted to heat said mandrel to a controlled degree.

6. The method for mutually bonding a plurality of coaxial metal tubes comprising placing said tubes in a closely fitting die body, positioning a tight fitting member Within the innermost of said tubes, and thermally expanding said member while maintaining the dimensions of said die body against thermal stress.

7. The method for mutually bonding a plurality of tight fitting coaxial metal tubes comprising fitting said coaxial tube assembly into tight registration in a cylindrical opening in a die member, fitting a cylindrical mandrel working member in tight registration into the cylindrical opening defined by the inner surface of said coaxial tube assembly, and thermally causing said mandrel working member to expand within said cylindrical opening in said die assembly While maintaining the dimensions of said die body against thermal stress.

8. The method for mutually bondin a plurality of tight fitting coaxial metal tubes, said method comprising fitting a plurality of metal tubes into juxtaposition so as to form a tight fitted coaxial tube assembly, fitting said coaxial tube assembly into tight registration in an insulation lined cylindrical opening in a die member, fitting a cylindrical mandrel working member in tight registration into the cylindrical opening defined by the inner surface of said coaxial tube assembly, passing cooling fluid through means disposed to conduct heat away from said die member, and heating said mandrel working member to just below the melting point of the material of the exterior tubes for one hour.

9. The method for mutually bonding a uranium fuel element core tube to a cladding tube on said core tube inner surface and to a second cladding tube on said core tube outer surface, said method comprising fitting a cladding tube tightly inside a uranium core Itube, fitting a second cladding tube tightly around the outside of said uranium core tube so as to form a coaxial three layered tubular fuel element assembly, fitting said fuel element assembly into tight registration in an insulatio =1 lined cylindrical opening in a die member, fitting a cylindrical mandrel working member in tight registration into the cylindrical opening defined by the inner surface of said fuel element assembly, passing cooling fiuid through means disposed to conduct heat away from said die member, heating said mandrel Working member to a temperature just below the melting point of said cladding element for one hour by passing electric current through electric heating coil-s contained in a hollow central portion of said working member, and shutting off said electric current, cooling said mandrel to ambient, and removing said bonded coaxial tube assembly.

10. The method for mutually bonding a plurality of 6 tight fitting coaxial metal tubes comprising fitting a plurality of metal tubes into juxtaposition so as to form a tight fitted coaxial tube assembly, placing said tubes in a close fitting die body, positioning a tight fitting mandrel in the innermost of said tubes, and thermally expanding said mandrel more than said die body.

ll. The method for mutually bonding a plurality of tight fitting coaxial metal tubes comprises fitting a plurality of metal tubes into juxtaposition so as to form a tight fitted coaxial tube assembly, fitting said coaxial tube assembly into tight registration in a cylindrical opening in a die member, fitting a cylindrical mandrel Working member in tight registration into the cylindrical opening defined by the inner surface of said coaxial tube assembly, thermally causing said mandrel working member to expand more than said die assembly, and heating said coaxial tube assembly coincidentally to approximately just below the melting point of the exterior of said tubes.

12.. The method for mutually bonding a plurality of tight fitting metal tubes, said method comprising fitting a plurality of metal tubes into juxtaposition so as to form a tight fitted coaxial tube assembly, fitting said coaxial tube assembly into tight registration in an insulation lined cylindrical opening in a. die member fabricated of a low thermal coefficient of expansion material, fitting a cylindrical mandrel working member fabricated of a high thermal coefilcient of expansion material in tight registration into the cylindrical opening defined by the inner surface of said coaxial tube assembly, subjecting the entire assembly of die means, coaxial tube assembly, and mandrel means to a temperature just below the melting point of the material comprising the exterior tubes for one hour, subsequently removing said entire assembly from said heated environment, cooling said assembly to ambient, and removing said bonded coaxial tube assembly from said die assembly.

13. The method for mutually bonding a plurality of tight fitting coaxial metal tubes comprising restraining the outermost of said tubes, positioning a tight fitting member within the innermost of said tubes, and thermally expanding said member within said tubes.

14. The method for mutually bonding a plurality of tight fitting coaxial metal tubes comprising placing said tubes in a close fitting die body, positioning a tight fitting mandrel in the innermost of said tubes, and thermally expanding said mandrel more than said die body.

15. A device for mutually bonding a plurality of coaxially fitted metal tubes comprising a die having a hollow body portion closely fitting the outermost of said tubes, a tight fitting member disposed within the innermost of said tubes, and means adapted to thermally expand said tight fitting member relative to said die.

References Cited in the file of this patent UNITED STATES PATENTS 1,695,791 Y-unck Dec. 18, 1928 2,848,800 Maloney et al Aug. 26, 1958 2,850,798 Bowman et al. Sept. 9, 1958 

